makefile: Reconstruct Makefile system.

This commit does 5 things:
	Update the root Makefile;
	Create a sub-Makefile under ./c/;
	Add .PHONY for "all" in Makefile.common;
	Add .PHONY for "install" and "clean" in Makefile.rules;
	Update a path in Makefile.config2 so that Makefile(s) can
work properly.